In accordance with recent advances in technology, various types of personal computers (PCs), such as desktop computers and notebook computers, have been developed and are currently being sold on the market. End users may purchase PCS having the standard components (e.g., a main processor, a main memory and other basic devices) for providing the basic system configuration. Additionally, end users may enhance or extend the capabilities of his or her PC by installing various peripheral devices.
Peripheral units referred to as "extended adaptor cards" are used to provide PCs with expanded capabilities. Unfortunately, extended adapter cards typically do not satisfy the constraints (e.g., weight limitations and high component density) required to produce compact, light, portable notebook computers. As such, a peripheral unit referred to as a "PC card", was developed which has the capability to expand the system configuration of portable notebook computers. PC cards are adapted to notebook computers that include a connector for the electrical connection of a PC card, and a PC card slot large enough to retain a PC card (see FIG. 5).
The existing mechanical and electrical specifications of the PC card were defined primarily by the PCMCIA (Personal Computer Memory Card International Association) and the JEIDA (Japan Electronic Industry Development Association). Presently, the PC card specification defines three types of PC cards: Type I, which is 3.3 mm thick; Type II, which is 5.5 mm thick; and Type III, which is 10.5 mm thick. Type I cards are used primarily as memory cards. Type II cards are used primarily as FAX/modem cards, Ethernet adaptor cards, and SCSI (Small Computer System Interface) adaptor cards. Type III cards are used primarily as cards that have built-in hard disks.
The PC card specification, released by the PCMCIA/JEIDA in 1995, defines the CardBus, and DMA (Direct Memory Access) support and multifunction cards, while maintaining compatibility with the prior PCMCIA Release 2.1 (JEIDA Version 4.2). The CardBus extends the internal bus width of a PC card from the conventional 16 bits to 32 bits, and increases the maximum clock speed to 33 MHZ and the maximum transfer rate to 132 Mbps. The CardBus is intended to be directly connected to a PCI (Peripheral Component Interconnect) bus which is a local bus for a computer (hereinafter referred to as a "host").
By adopting the CardBus, highly functional PC cards can be achieved, such as high speed Ethernet cards, high speed memory cards, and multimedia cards that can handle graphics and moving pictures. The CardBus typically requires a PC card that incorporates an MPU such that the PC card can serve as a bus master, and can obtain the right for controlling the system bus of the host. This differs greatly from a conventional PC card that serves merely as a bus slave.
Many PC cards of the previous version also incorporate an MPU for controlling the internal operation of the PC card, or for providing a cooperative transaction, such as data exchange, with the host computer.
One problem encountered in designing a PC card which includes a general purpose MPU, rather than a special purpose LSI (Large Scale Integrated) circuit whose applications are specific, is that the power consumption may be increased due to a larger number of gates in an MPU. However, under certain circumstances, it may be desirable to use an MPU rather than a special purpose LSI, despite the possible increase in power consumption. For example, the use of an MPU is typically required when the special purpose LSI is not capable of supporting all the functional capabilities of the PC card. Thus, reducing the power consumption of a PC card which includes an MPU is a concern for PC card designers, especially if the PC card is used in conjunction with a host computer that is a battery operated notebook computer.
One method for reducing the power consumption (power management) in a conventional PC card is to collectively halt the operating clocks for an MPU and its peripheral circuits after a series of operations has been completed. With this method, however, the MPU is driven continuously during a period when the PC card is operating at a more or less reduced capacity, such as during a period when data is being exchanged with a computer system. So long as the MPU, which requires a large amount of power, is driven continuously, the amount of saved power is limited.